This invention relates to an aqueous alkaline cell with a cathode mixture comprising copper iodate, particularly copper iodate and additive of sulfur or graphitic carbon nanofiber.
Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, polymeric binders, and other additives, such as titanium-containing compounds can be added to the cathode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The resistivity of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3xc3x9710xe2x88x923 ohm-cm and 4xc3x9710xe2x88x923 ohm-cm.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
Although such alkaline cells are in widespread commercial use there is a need to improve the cell or develop a new type of cell that is cost effective and exhibits reliable performance and even longer service life for normal applications such as flashlight, radio, audio recorders and portable CD players.
The invention is directed to a primary (nonrechargeable) electrochemical alkaline cell having an anode comprising zinc and a cathode mixture comprising copper iodate Cu(IO3)2. The anode and cathode include an aqueous alkaline solution, preferably aqueous KOH solution. The copper iodate is preferably in the form of a powder having an average particle size between about 1 and 100 micron. The cathode mixture preferably also includes a sulfur additive. The additive is preferably sulfur but can also be elements selenium (Se) or tellurium (Te) and mixtures thereof. The sulfur additive enhances the cell performance, that is, elevates the running voltage of the cell, which in turn leads to increased power and cell life. The cathode mixture includes a conductive material such as flaky crystalline natural graphite or flaky crystalline synthetic graphite including expanded graphite and graphitic carbon nanofibers. The term graphitic carbon nanofibers as used herein shall mean graphitic carbon fibers having a mean average diameter less than 1000 nanometers (less than 1000xc3x9710xe2x88x929 meter). Preferably, the graphitic carbon nanofibers have a mean average diameter less than 500 nanometer, more preferably less than 300 nanometers. Desirably the graphitic carbon nanofibers have a mean average diameter between about 50 and 300 nanometers, typically between about 50 and 250 nanometers. The cathode mixture includes an aqueous KOH solution, desirably having a concentration of between about 30 and 40 percent by weight, preferably between 35 and 45 percent weight KOH in water.
The copper iodate preferably comprises between about 82 and 90 percent by weight of the cathode mixture. The graphitic conductive material, preferably comprising graphitic carbon nanofibers, desirably comprises between about 4 and 10 percent by weight of the cathode mixture. The sulfur additive desirably comprises between about 5 and 10 percent by weight of the cathode mixture. The aqueous KOH solution desirably comprises between about 5 and 10 percent by weight of the cathode mixture.
The alkaline cell of the invention having an anode comprising zinc and a cathode mixture comprising copper iodate exhibits high capacity (mAmp-hrs) under a moderate high current density (e.g. 21 milliAmp/cm2 based on anode/cathode interface) when compared to conventional alkaline cells having an anode comprising zinc and cathode comprising manganese dioxide.